Document way of contributing new built-in templates

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gitlab-ce!26976 Introduces a link to https://gitlab.com/gitlab-org/project-templates/contributing as official way to contribute built-in templates As @lbennett suggests linking directly to repo may become an issue if we reorganize things in future - linking to documentation will be more reliable way to handle this link.

When this issue will be solved we should update link in gitlab-ce code to point to documentation instead

Other links/references

gitlab-ce!26976 gitlab-ce#55833